1. Field of the Invention
This invention relates generally to sources of electromagnetic laser radiation by semiconductor lasers and, in particular, to vertical cavity surface emitting lasers (VCSELs) and methods for fabricating same.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors, one of which serves as the xe2x80x9cexitxe2x80x9d mirror. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through one of the mirrors (the exit mirror) as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optical pumping (OP) or electron beam pumping. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region where recombination of electron and holes occurs. There are two kinds of recombination events, i.e. radiative and non-radiative, concurrently happening in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states. Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) If a sufficient amount of radiative recombinations are stimulated by photons, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The xe2x80x9cverticalxe2x80x9d direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with xe2x80x9cupxe2x80x9d being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror. The exit mirror typically has slightly lower reflectance (i.e., reflectivity) than the other mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active region sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. Both EP and OP VCSEL designs are possible. The two mirrors may be referred to as a top DBR and a bottom DBR. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectance in order to achieve a sufficient level of feedback for the device to lase. DBRs are typically formed of multiple pairs of layers referred to as mirror pairs; DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction; for semiconductor DBRs, the layers are typically selected so that they are easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques.
When properly designed, these mirror pairs will cause a desired reflectance at the lasing wavelength, at which wavelength the active region is also designed to have sufficient gain to permit lasing to occur. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectance, while the top (exit) DBR mirror has a reflectance that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. In other designs, the bottom mirror may serve as the exit mirror and the top mirror has the higher reflectance.
For semiconductor DBRs, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectance, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectance). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectance to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectance as a larger number in a semiconductor DBR. However, semiconductor DBRs, despite their lower reflectance/greater thickness, can be preferred because of comparative advantages in electrical conductivity, thermal conductivity, and manufacturability For example, in an EP VCSEL, a semiconductor DBR can be preferred, especially for the bottom DBR (between the substrate and active region), to conduct the pumping current through the active region, the bottom DBR, and into the substrate. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., it may be required if the initial epitaxial substrate is to be used for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR). Accordingly, there is often a tradeoff between using a lower reflectance, thicker semiconductor DBR, or a higher reflectance, thinner dielectric DBR which is more difficult to manufacture or which makes thermal conductivity more of an issue.
VCSEL technology and related matters are discussed in further detail in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin and Larry A. Coldren, Cambridge: Cambridge University Press (1999); U.S. Pat. No. 5,468,656, Shieh et al., xe2x80x9cMethod of Making a VCSELxe2x80x9d; U.S. Pat. No. 5,985,686, Jayaraman, xe2x80x9cProcess for Manufacturing Vertical Cavity Surface Emitting Lasers Using Patterned Wafer Fusion and the Device Manufactured by the Processxe2x80x9d; and MacDougal et al., xe2x80x9cElectrically-Pumped Vertical-Cavity Lasers with AlO-GaAs Reflectorsxe2x80x9d, IEEE Photonics Letters, vol. 8, No. 3, March 1996.
It is desirable to employ long-wavelength VCSELs, e.g. those having emission in the infra-red spectrum (e.g., wavelengths longer than 1 xcexcm (micron)), or other long-wavelength outputs such as 1.3 xcexcm (i.e., 1310 nm) to 1.55 xcexcm. Long wavelength (1.3 xcexcm to 1.55 xcexcm) VCSELs are also of great interest in the optical telecommunications industry because of the minimum fiber dispersion at 1.32 xcexcm and the minimum fiber loss at 1.55 xcexcm. The dispersion shifted fiber will have both minimum dispersion and minimum loss at 1.55 xcexcm. Other long wavelength, infrared VCSELs can be employed in various medical applications.
The typical reflectance that is required for the bottom cavity mirror of a high performance VCSEL is more than 99.5%, especially for longer wavelength VCSELs which have lower gain compared to shorter wavelength ones (in general, where there is lower gain, higher cavity reflectance is needed). Long wavelength VCSELs are often based on an InP-based material system, e.g. an InxGal-xASyPl-y active layer lattice matched to InP cladding layers. The semiconductor DBRs used in such VCSELs have to be lattice matched to the VCSEL""s material system. Unfortunately, in the available (i.e., lattice matched) semiconductor DBR material systems for such VCSELs, there are typically very small difference in the refractive indices in this material system. This makes it difficult to achieve the desired reflectance, without employing a large number of mirror pairs, which leads to often unacceptably thicker DBRs. Moreover, the greater the operating (emitting) wavelength of the laser, the thicker each layer must be, further contributing to the increased thickness of long-wavelength semiconductor DBRs.
Long-wavelength VCSELs thus would require a comparatively thick semiconductor bottom DBR, which can be difficult to manufacture. Such thick DBRs can also have poor thermal conductivity, so that it is difficult to achieve adequate heat dispersion to the heat-spreading submount to which the bottom DBR is mounted. The formation of the thick semiconductor DBR on an InP substrate, for example, causes serious manufacturability and other problems, as described above.
Many attempts have been made to address this problem, including fabrication of devices that utilize wafer bonding techniques, but only limited success has been achieved. As an example, devices are currently utilized in which a semiconductor DBR mirror structure is epitaxially grown on a GaAs substrate. Next, the active layer is grown on the InP substrate. The two elements are then flip chip mounted together and fused using wafer fusion techniques. The end result is a device that is expensive to manufacture, and which exhibits low efficiency, low output power, and low yield. In addition, the interface defect density in the wafer fusion procedure causes potential reliability problems of the VCSEL end product.
There is, therefore, a need for improved VCSEL mirrors and methods for fabricating same. In particular, it is desirable to improve the reflectance of the bottom cavity mirrors for long-wavelength VCSELs.